Simplify the following expression: $z = \dfrac{10}{9r} + \dfrac{3}{10r}$
Answer: In order to add expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $9r$ and $10r$ $\lcm(9r, 10r) = 90r$ $ z = \dfrac{10}{10} \cdot \dfrac{10}{9r} + \dfrac{9}{9} \cdot \dfrac{3}{10r} $ $z = \dfrac{100}{90r} + \dfrac{27}{90r}$ $z = \dfrac{100 +27}{90r}$ $z = \dfrac{127}{90r}$